Field of the Invention
The invention relates to a device for forming an image on a printing plate having at least one laser and an optical system for forming an image of laser radiation on the printing plate.
It has been known for some time that it is possible to form an image on a printing plate, whether the latter has a planar or curved surface, by irradiating the surface thereof with intensive laser radiation. A physical or chemical change in the surface properties occurs as a result of the light/material interaction. In the case wherein the surface properties are changed by the thermal effect of the laser radiation, a specific threshold energy density is necessary for producing a dot. The density depends, amongst other things, on the material parameters of the printing plate and the time duration of the irradiation. If the energy density is lower than the threshold energy density, then no dot is produced even if exposure occurs over a very long period of time. Typically, the threshold energy density decreases with decreasing time duration of the irradiation by the laser.
For forming images on printing plates, radiation which is generated during continuous operation of the laser, so-called continuous-wave operation, is used in many realized applications. The time duration of the irradiation by the laser is typically determined by the laser oscillation being switched on and off or the beam being interrupted, with the result that exposures in a range of microseconds or greater typically occur. A shorter time duration of the exposure can MD be achieved by using lasers which emit pulses. Q-switched lasers in pulse operation are proposed for a series of applications. This generally involves gas laser or solid-state laser systems.
U.S. Pat. No. 5,874,981 discloses how, by modulation of the energy supply of the light source that is used, amplitude and time modulation of the laser radiation that is generated can be effected, with the result that an image is produced on a surface. In this regard, the laser is used for short periods of time in continuous-wave operation.
The published German Patent Document DE 195 44 502 C1 describes a laser engraving installation. A modulated laser beam is used to form a desired profile in a workpiece surface. In this regard, the fine structures of the profile are formed by the beam of a first laser, which is amplitude-modulated by an acousto-optical modulator with a relatively high modulation frequency in the MHz range, while the deep regions of the desired profile are formed by the beam of a second laser. The modulator and the second laser radiation source are driven by mutually related, but separate control signals. The lasers are used for short periods of time in continuous-wave operation.
U.S. Pat. No. 5,208,819 describes a laser system for recording data patterns on a surface. A light modulator is exposed by the pulsed laser radiation of an excimer laser, with the result that a pattern can be projected onto a surface. The light modulator includes an array of deformable mirrors which can thus be switched between an activated and a deactivated state. In U.S. Pat. No. 5,940,115, a laser system is used which emits pulses in the microseconds range. This typically involves a gas laser, in particular a CO2 laser. The laser pulses are used to write dots on a photosensitive material. The imaging onto the surface is effected by a reducing optical arrangement so that only the light reflected from the activated mirrors falls onto the surface.
U.S. Pat. No. 3,657,510 presents a Q-switched laser for altering surfaces. What is involved, in this regard, is an optically pumped laser, preferably a solid-state laser. The laser pulses generated by the Q-switching serve for forming an image of a mask, which is situated within the laser resonator, onto a surface. The irradiation with laser light results in an alteration of the surface, for example, by evaporation, heating, chemical reaction or oxidation.
For forming an image on a printing plate, 0.5 J/cm2 is typically necessary as threshold energy density in continuous-wave operation. If the dot size is about 10 micrometers, a threshold energy of 0.5 to 3 xcexcJ thus results. For image-forming using a diode laser, therefore, an output power of 100 to 500 mW is necessary per individual beam. The high optical power that is required necessitates a corresponding electrical power. It typically amounts to three watts per individual beam. As a consequence, corresponding cooling is necessary. Complicated air or water cooling makes it difficult to integrate the image-forming device in a compact form.
Gas laser or solid-state laser systems are less suitable for practical use in devices for forming an image on a printing plate, in particular in printing units or printing machines. Such systems require a complicated pump device for generating the laser oscillation, and typically have a large construction space mass and are expensive. Physical limits are imposed on the minimum pulse duration that can be achieved when generating pulses by Q-switched laser systems; minimum pulse durations are typically a few 10xe2x88x928 seconds.
In view of the foregoing, it is an object of the invention, therefore, to provide an improved device for forming images on printing plates using radiation emitted by a laser, which may serve to achieve a lower threshold energy density.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for forming an image on a printing plate, comprising at least one laser, and an optical system for forming an image of radiation from the laser on the printing plate, the laser radiation having ultrashort pulses with a duration of less than 1 ns.
In accordance with another feature of the invention, the laser radiation is generatable by a semiconductor laser.
In accordance with a further feature of the invention, the laser is installed for multimode operation and is mode-coupled.
In accordance with an added feature of the invention, the one laser has a supply current with ac and dc components.
In accordance with an additional feature of the invention, the one laser is an individual diode laser for generating the laser radiation.
In accordance with yet another feature of the invention, the at least one laser is a diode laser array for generating the laser radiation.
In accordance with yet a further feature of the invention, the diode laser array comprises individually drivable single-strip diode lasers.
In accordance with yet an added feature of the invention, the device for forming an image on a printing plate includes a control arrangement for temperature regulation of the laser.
In accordance with yet an additional feature of the invention, the optical system for forming an image of the radiation on the printing plate has reflective elements.
In accordance with still another feature of the invention, the optical system has micro-optical elements.
In accordance with still a further feature of the invention, the dc-component is able to be modulated.
In accordance with an added aspect of the invention, there is provided a printing unit having at least one device for forming an image on a printing plate, comprising at least one laser, and an optical system for forming an image of radiation from the laser on the printing plate, the laser radiation having ultrashort pulses with a duration of less than 1 ns.
In accordance with a concomitant aspect of the invention, there is provided a printing machine, comprising at least one printing unit having at least one device for forming an image on a printing plate, the device including at least one laser, and an optical system for forming an image of radiation from the laser on the printing plate, the laser radiation having ultrashort pulses with a duration of less than 1 ns.
The nonlinear dependence of the threshold energy density of thermal printing plates on the temporal pulse width of the laser radiation becomes clear for ultrashort pulses. At a pulse width of 10 ps, for example, a threshold energy density of 0.02 J/cm2 results. This threshold energy density is a factor of 25 less than that in continuous-wave operation of the laser. In order to generate laser light pulses with a temporal width of a few nanoseconds to picoseconds, in particular, the method of mode coupling is known in the literature. See, for example, P. W. Milonni and J. H. Eberly, xe2x80x9cLasersxe2x80x9d, Wiley, New York, N.Y., 1988. Such a method can also be used in the case of diode lasers for generating short light pulses. Note, for example, P. Vasil""ev, xe2x80x9cUltrafast diode lasersxe2x80x9d, Artechhouse Inc., 1995.
Through the use of a laser which emits ultrashort pulses with a duration of less than 1 ns in the device, a lower average power is necessary, compared with continuous-wave operation, for the imaging per individual beam. In a preferred embodiment, a semiconductor laser is involved in this case. For pumping a pulsed laser, a lower electrical power is required during operation. Therefore, less cooling is required, with the result that the corresponding device can be configured more simply. Consequently, it is simpler to realize compact image-forming devices in integrated form. Furthermore, the lower thermal loading increases the service life of the lasers.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in as an imaging device for forming an image on a printing plate by using ultrashort laser pulses, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: